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Publication numberUS4857858 A
Publication typeGrant
Application numberUS 07/186,285
Publication dateAug 15, 1989
Filing dateApr 26, 1988
Priority dateApr 30, 1987
Fee statusLapsed
Also published asCA1280472C, DE3886444D1, DE3886444T2, EP0289037A2, EP0289037A3, EP0289037B1
Publication number07186285, 186285, US 4857858 A, US 4857858A, US-A-4857858, US4857858 A, US4857858A
InventorsMasato Tahara
Original AssigneeNec Corporation
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Digital demodulation system having independently operating cross-polarization cancellers
US 4857858 A
Abstract
In a digital radio transmission in which first- and second-channel radio frequency digital signals are transmitted respectively on othogonally polarized radiowaves and first and second IF signals are derived respectively from the first- and second-channel RF signals. The second IF signal is proportioned in response to a control signal and subtractively combined with the first IF signal. A main digital demodulator provides demodulation on the combined IF signals to generate a main (N+1)-bit output and derives an N-bit data signal from N higher significant bits of the (N+1)-bit output and an error component from the LSB of the main (N+1)-bit output. An auxiliary digital demodulator provides demodulation of the second IF signal to generate an auxiliary (N+1)-bit output and derives a data component of the second channel from the MSB of the auxiliary (N+1)-bit output. The derived error and data components are supplied to a correlator to generate the control signal.
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Claims(4)
What is claimed is:
1. A demodulation system for digital radio transmission in which first- and second-channel radio frequency (RF) digital signals are transmitted respectively on orthogonally polarized radiowaves and intermediate frequency (IF) versions of said first- and second-channel radio frequency signals are derived from said radio frequency digital signals, said system comprising:
variable coupling means for proportioning the IF version of said second-channel RF signal in response to a control signal applied thereto and subtractively combining the proportioned IF version of said second-channel RF signal with the IF version of said first-channel RF signal;
main digital demodulator means for demodulating an output of said variable coupling means to generate a main (N+1)-bit output and deriving an N-bit information signal as a data component of said first-channel RF signal from N higher significant bits of said main (N+1)-bit output and an error component of said first-channel RF signal from the least significant bit of the main (N+1)-bit output;
auxiliary digital demodulator means for demodulating the IF version of said second-channel RF signal to generate an auxiliary (N+1)-bit output and deriving a data component of said second-channel RF signal from the most significant bit of said auxiliary (N+1)-bit output; and
means for determining a correlation between said error component and said data component and deriving therefrom said control signal.
2. A demodulation system as claimed in claim 1, further comprising:
second variable coupling means for proportioning the IF version of said first-channel RF signal in response to a second control signal applied thereto and subtractively combining the proportioned IF version of said first-channel RF signal with the IF version of said second-channel RF signal;
second main digital demodulator means for demodulating an output of said second variable coupling means to generate a second main (N+1)-bit output and deriving an N-bit information signal as a data component of said second-channel RF signal from N higher significant bits of said main (N+1)-bit output and an error component of said second-channel RF signal from the least significant bit of said second main (N+1)-bit output;
second auxiliary digital demodulator means for demodulating the IF version of said first-channel RF signal to generate a second auxiliary (N+1)-bit output and deriving a data component of said first-channel RF signal from the most significant bit of said second auxiliary (N+1)-bit output; and
means for determining a correlation between said error component of said second-channel RF signal and said data component of said second-channel RF signal and deriving therefrom said second control signal.
3. A demodulation system for digital radio transmission in which first- and second-channel radio frequency (RF) digital signals are transmitted respectively on orthogonally polarized radiowaves and intermediate frequency (IF) versions of said first- and second-channel radio frequency signals are derived from said radio frequency digital signals, said system comprising:
variable coupling means for proportioning the IF version of said second-channel RF signal in response to a control signal applied thereto and signal with the IF version of said first-channel RF signal;
main digital demodulator means for demodulating an output of said variable coupling means to generate a main (N+1)-bit output and deriving an N-bit information signal as a data component of said first-channel RF signal from N higher significant bits of said main (N+1)-bit output;
auxiliary digital demodulator means for demodulating the IF version of said second-channel RF signal to generate an auxiliary (N+1)-bit output; and
means for deriving a data component of said second-channel RF signal from the most significant bit of said auxiliary (N+1)-bit outputs, deriving an error component of said first-channel RF signal from the least significant bit of said main (N+1)-bit outputs, determining a correlation between said error component and said data component, and deriving said control signal from the determined correlation.
4. A demodulation system as claimed in claim 3, further comprising:
second variable coupling means for proportioning the IF version of said first-channel RF signal in response to a second control signal applied thereto and subtractively combining the proportioned IF version of said first-channel RF signal with the IF version of said second-channel RF signal;
second main digital demodulator means for demodulating an output of said second variable coupling means to generate a second main (N+1)-bit output and deriving an N-bit information signal as a data component of said second-channel RF signal from N higher significant bits of said second main (N+1)-bit output;
second auxiliary digital demodulator means for demodulating the IF version of said first-channel RF signal to generate a second auxiliary (N+1)-bit output; and
means for deriving a data component of said first-channel RF signal from the most significant bit of said second auxiliary (N+1)-bit outputs, deriving an error component of said second-channel RF signal from the least significant bit of said main (N+1)-bit outputs, determining a correlation between said error component of said second-channel RF signal and said data component of said second-channel RF signal, and deriving said second control signal from the determined correlation between the last-mentioned error and data components.
Description
BACKGROUND OF THE INVENTION

The present invention relates generally to digital demodulation systems for demodulating intermediate frequency (IF) versions of radio frequency signals carried respectively on orthogonally oriented polarization planes, and more specifically to cancellation of interference between the polarization planes.

To achieve frequency utilization it has been a common practice to transmit two digital signals on respective channels formed by orthogonally oriented polarization planes, either vertical-horizontal or clockwise-counterclockwise orientation. However, fading and anisotropic nature of the transmission medium are the potential source of troubles, causing cross-polarization interference between the orthogonal planes. As shown and described in U.S. Pat. No. 4,575,862 issued to M. Tahara et al and assigned to the same assignee as the present invention and a paper "IF Band Cross-Polarization Canceler" by T. Ryu, M. Tahara and T. Noguchi, IEEE International Conference on Communications, Proceedings, Volume 2 May 14-17, 1984, Amsterdam, prior art cross-polarization cancellation involves the use of a single digital demodulator for each channel to process the intermediate frequency (IF) version of a radio frequency signal. The demodulator includes an (N+1)-bit analog-to-digital converter. The higher significant N bits of the (N+1)-bit output of the converter represent the original N-bit code and the LSB (least significant bit) of the (N+1)-bit converter output, which represents the cross-polarization interference, is supplied as an error component to a correlator of the own channel, while the MSB (most significant bit) of the (N+1)-bit converter output is cross-coupled to the correlator of the other channel as a data component. The correlator of each channel generates a control signal with which the amplitude of the IF signal of the other channel is proportioned and subtractively combined with the IF signal of the own channel, the combined IF signals being fed to the digital demodulator of the own channel to cancel the cross-polarization interference contained in the IF signal of the own channel. Since each demodulator forms part of the closed loop of the other channel, a modification of the operating parameters of one results in an alteration of the operating parameters of the other channel equipment.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a cross-polarization digital demodulation system in which the modification of one channel has no effect on the other channel.

The digital demodulation system embodying the present invention is adapted for digital radio transmission in which first- and second-channel radio frequency digital signals are transmitted respectively on orthogonally polarized radiowaves and first and second intermediate frequency (IF) versions of the first- and second-channel radio frequency signals are derived. Each of these IF versions of the radio signals is applied to a variable coupler in which it is proportioned in amplitude in response to a control signal applied thereto and subtractively combined with the IF version of the other channel. Main and auxiliary digital demodulators are provided to recover the original N-level analog signals and derive (N+1)-bit outputs from these analog signals. The main demodulator provides demodulation on the output of the variable coupler and recovers the original N-bit information signal from the higher bits of its (N+1)-bit output and deriving an error component from the LSB of the (N+1)-bit output indicating the cross-polarization interference between the first- and second-channel radio frequency signals. The auxiliary digital demodulator provides demodulation on the IF version of the other channel to derive a data component from the MSB of its (N+1)-bit output. Correlation between the error component and the data component developed by the main and auxiliary demodulators is determined, and from this determination the control signal is generated to minimize the cross-polarization interference introduced to the own channel.

Due to the provision of the auxiliary digital demodulator, the closed loop interference cancellation of each channel operates independently from the other channel, and hence the present invention no longer suffers from the problems associated with the prior art system.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described in further detail with reference to the accompanying drawings, in which:

FIG. 1 is a block diagram of a prior art cross-polarization canceller; and

FIG. 2 is a block diagram of a cross-polarization canceller of the present invention.

DETAILED DESCRIPTION

Before describing the detail of the present invention, it is appropriate to describe a prior art digital demodulation system having a cross-polarization canceller with reference to FIG. 1. The prior art demodulation system generally comprises first- and second-channel receiving circuits 1 and 2 of identical configuration for respectively receiving through terminals 101 and 102 first- and second-channel intermediate frequency (IF) versions of digitally modulated radiowaves such as 16 or 64 QAM (quadrature amplitude modulation) signals which have been carried on orthogonally polarized planes, either vertical-horizontal orientation or clockwise-counterclockwise orientation.

Each of the receiving circuits 1 and 2 comprises a variable coupler 3, a digital demodulator 4 and a correlator 5. Each variable coupler 3 is formed by a subtracter 6 and a transversal filter 7. The subtracter 6-1 of the first receiving circuit 1 combines the IF version of the first-channel signal at terminal 101 with the IF version of the second-channel signal from terminal 102 which is proportioned in amplitude by transversal filter 7-1 in response to a control signal supplied from the correlator 5-1. Likewise, the subtracter 6-2 of the second receiving circuit 2 combines the IF version of the second-channel signal at terminal 102 with the IF version of the first-channel signal which is proportioned in amplitude by transversal filter 7-2 in response to a control signal supplied from the correlator 5-2. Digital demodulator 4-1 includes a quadrature demodulator that provides digital demodulation on the output of subtracter 3-1 to generate an output which is a replica of the original first-channel baseband N-level signal. Demodulator 4-1 further includes an (N+1)-bit analog-to-digital converter which converts the N-level analog signal into an (N+1)-bit signal by comparing it with predetermined thresholds. If the incoming signals are of 16 QAM format, the demodulated signal is a four-level signal which is converted into a 3-bit code and a 2-bit main data signal is derived from the higher significant 2-bits of the 3-bit code. This 2-bit main data signal is applied to an output terminal 103 and the most significant bit of the code is applied to the other correlator 5-2. The least significant bit of the 3-bit code represents an error component indicating the second-to-first channel cross-polarization interference, this LSB being applied to the own correlator 5-1. In a symmetrical manner, demodulator 4-2 derives a 2-bit main data signal from the higher significant bits of a 3-bit output and applied to an output terminal 104, the MSB of the 3-bit output being applied to the other correlator 5-1 as a data component and the LSB of the 3-bit output being applied to the own correlator 5-2 as an error component indicating the first-to-second channel cross-polarization interference. Each of the first- and second-channel correlators 5-1 and 5-2 detects the amount of correlation between the interfering component derived from the own channel signal and the data component derived from the demodulator of the other channel and generates a control signal that controls the tap weights of transversal filter 7 of the own channel to suppress the cross-polarization interference. Therefore, interference cancellation is provided by cross-coupled feedback operations.

One disadvantage of the prior art canceller, however, lies in the fact that, as a result of the cross-coupling between the two channels, a faulty condition in one of the two channels has caused a trouble or abnormality in the other channel. Another disadvantage is that, if the system is initially started only with one channel, the starting of the other channel at a later date will necessitate the initialization of the existing channel.

Referring to FIG. 2, a demodulation system according to this invention is illustrated. Each of the first- and second-channel receiving circuits, designated 21 and 22 respectively, includes a variable coupler 30, a main digital demodulator 40, a correlator 50 and an auxiliary digital demodulator 80. Variable coupler 30 comprises a subtracter 60 and a transversal filter 70 whose output is connected to an input of the subtracter 60. The first-channel IF signal is applied through terminal 201 to the subtracter 60-1 of the own channel and to the transversal filter 70-2 and auxiliary demodulator 80-2 of the second-channel receiver 22, and likewise, the second-channel IF signal is applied through terminal 202 to the subtracter 60-2 of the own channel and to the transversal filter 70-1 and auxiliary demodulator 80-1.

Main demodulator 40-1 comprises a quadrature demodulator for demodulating the output of the subtracter 60-1 to derive an N-level analog signal and an (N+1)-bit analog-to-digital converter for deriving a main (N+1)-bit output from the N-level analog signal. An N-bit main data signal is taken from the N higher significant bits of the main (N+1)-bit output and supplied to an output terminal 203. The least significant bit of the main (N+1)-bit output is supplied to the correlator 50-1 as an error component indicating the second-to-first channel cross-polarization interference. Auxiliary demodulator 80-1 is identically constructed to the main demodulator 40-1 and demodulates the second-channel IF signal into an N-level analog signal and converts it into an auxiliary (N+1)-bit output and derives a data component of the second channel from the most significant bit of the auxiliary (N+1)-bit output, this main data component being supplied to the correlator 50-1.

In a similar manner, main demodulator 40-2 demodulates the output of the subtracter 60-2 to detect an N-level analog signal and derives a main (N+1)-bit output from the N-level analog signal. The N higher significant bits of the main (N+1)-bit output are supplied to an output terminal 204 and the least significant bit of the main (N+1)-bit output is supplied to the correlator 50-2 as an error component indicating the first-to-second channel cross-polarization interference. Auxiliary demodulator 80-2 is identically constructed to the main demodulator 40-2 and derives from the first-channel IF signal an auxiliary (N+1)-bit output and supplies the MSB of this output to the correlator 50-2 as a data component of the first channel.

Each of the first- and second-channel correlators 50-1 and 50-2 detects the amount of correlation between the interfering component derived from the own channel and the MSB component that is derived from the other channel through the auxiliary demodulator of the own channel and controls the tap weights of transversal filter 70 of the own channel so that the cross-polarization interference of interest is suppressed by the subtracter 60 and hence error-free multibit codes can be obtained at the output terminals 203 and 204.

Since the auxiliary demodulators 80 do not form part of the closed loop of the other channels, the problems mentioned earlier in connection with the prior part demodulation system can be eliminated.

The foregoing description shows only one preferred embodiment of the present invention. Various modifications are apparent to those skilled in the art without departing from the scope of the present invention which is only limited by the appended claims. Therefore, the embodiment shown and described is only illustrative, not restrictive.

Patent Citations
Cited PatentFiling datePublication dateApplicantTitle
US4575862 *Dec 20, 1983Mar 11, 1986Nec CorporationCross-polarization distortion canceller for use in digital radio communication receiver
US4688235 *Nov 27, 1985Aug 18, 1987Nec CorporationCross-polarization interference canceller
Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US4965809 *Jun 28, 1989Oct 23, 1990Nec CorporationUplink cross-polarization interference canceller using correlation calculator and stepwise tracking controller
US5822424 *Nov 12, 1996Oct 13, 1998Samsung Electronics Co., Ltd.Dial pulse detector and detecting method for paging service for mechanical telephone subscriber
US7046753Jun 29, 2004May 16, 2006Provigent Ltd.Interference canceller with fast phase adaptation
US7613260Nov 21, 2005Nov 3, 2009Provigent LtdModem control using cross-polarization interference estimation
US7643512Jun 29, 2006Jan 5, 2010Provigent Ltd.Cascaded links with adaptive coding and modulation
US7720136Dec 26, 2006May 18, 2010Provigent LtdAdaptive coding and modulation based on link performance prediction
US7796708Mar 29, 2006Sep 14, 2010Provigent Ltd.Adaptive receiver loops with weighted decision-directed error
US7821938Apr 20, 2007Oct 26, 2010Provigent Ltd.Adaptive coding and modulation for synchronous connections
US7839952Dec 5, 2006Nov 23, 2010Provigent LtdData rate coordination in protected variable-rate links
US8001445Aug 13, 2007Aug 16, 2011Provigent Ltd.Protected communication link with improved protection indication
US8040985Oct 9, 2007Oct 18, 2011Provigent LtdDecoding of forward error correction codes in the presence of phase noise
US8315574Apr 13, 2007Nov 20, 2012Broadcom CorporationManagement of variable-rate communication links
US8351552Oct 17, 2011Jan 8, 2013Provigent Ltd.Decoding of forward error correction codes in the presence of phase noise and thermal noise
US8364179Mar 31, 2010Jan 29, 2013Provigent Ltd.Feedback-based management of variable-rate communication links
US8385839Mar 31, 2010Feb 26, 2013Provigent Ltd.Message-based management of variable-rate communication links
WO2001078090A1Apr 6, 2001Oct 18, 2001Astec Int LtdPlanar transformer
Classifications
U.S. Classification329/311, 375/349
International ClassificationH04B7/00, H04J11/00
Cooperative ClassificationH04B7/002
European ClassificationH04B7/00B
Legal Events
DateCodeEventDescription
Oct 28, 1997FPExpired due to failure to pay maintenance fee
Effective date: 19970820
Aug 17, 1997LAPSLapse for failure to pay maintenance fees
Mar 25, 1997REMIMaintenance fee reminder mailed
Nov 9, 1992FPAYFee payment
Year of fee payment: 4
Apr 26, 1988ASAssignment
Owner name: NEC CORPORATION, 33-1, SHIBA 5-CHOME, MINATO-KU, T
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST.;ASSIGNOR:TAHARA, MASATO;REEL/FRAME:004867/0299
Effective date: 19880418
Owner name: NEC CORPORATION,JAPAN
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:TAHARA, MASATO;REEL/FRAME:004867/0299